People have developed a recording material that can have its color erased when heated over a prescribed temperature. When this recording material is adopted, by applying heat on the image printed on the paper sheet, the paper sheet can be reused. However, in order to reuse the paper sheet, it is necessary to check whether the color of the image is sufficiently erased over a certain level of soundness and whether the paper sheet is not broken. In the recent years, people have developed a decoloring system with the function in decoloring the color on the paper sheet and the function in determining whether the paper sheet can be reused.